Method of driving display device

ABSTRACT

An active matrix type EL display device is provided, which is capable of suppressing the unevenness of luminance display due to the unevenness of the characteristics of TFTs which constitute pixels, or due to variations in the environmental temperature at which the display device is used. The active matrix type EL display is driven by a time gray scale method, and is capable of keeping the drain current of each of its EL driving TFTs constant by operating each of the EL driving TFTs in a saturation region in an ON state. Accordingly, constant current can be made to flow in each of the EL elements, whereby it is possible to provide an active matrix type EL display device with accurate gray scale display and high image quality.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of driving anelectronic display device having EL (electro luminescence) elementsformed on a substrate. More particularly, the invention relates to amethod of driving an EL display device using semiconductor elements(elements using semiconductor thin films) as well as to electronicequipment of the type in which an EL display device is used as a displaypart.

[0003] Incidentally, the term “EL element” used herein indicates both anelement which uses emission from a singlet exciter (fluorescence) and anelement which uses emission from a triplet exciter (phosphorescence).

[0004] 2. Description of the Related Art

[0005] In recent years, in the field of self-emitting elements, thedevelopment of EL display devices having EL elements has been becomingmore and more active. EL display devices are called organic EL displays(OELD(s)) or organic light emitting diodes (OLED(s)).

[0006] Such an EL display device is of the self-emitting type whichdiffers from liquid crystal devices. An EL element has a structure inwhich an EL layer is interposed between a pair of electrodes (an anodeand a cathode), and ordinary EL layers have a stacked structure.Representatively, there is a stacked structure which is called “holetransport layer/light emitting layer/electron transport layer”, proposedby Tang et al. of Kodak Eastman Company. This structure has very highemission efficiency, and is adopted in nearly all EL display devicescurrently under research and development.

[0007] Other structures may also be adopted, such as a structure inwhich “a hole injection layer, a hole transport layer, a light emittinglayer and an electron transport layer” are stacked on an anode in thatorder, or a structure in which “a hole injection layer, a hole transportlayer, a light emitting layer, an electron transport layer and anelectron injection layer” are stacked on an anode in that order. Thelight emitting layer may also be doped with a fluorescent pigment or thelike.

[0008] All the layers provided between a cathode and an anode are hereingenerically called “EL layer”. Accordingly, all the aforementioned holeinjection layer, hole transport layer, light emitting layer, electrontransport layer and electron injection layer are encompassed in the ELlayer.

[0009] When a predetermined voltage is applied across a pair ofelectrodes (both electrodes) of the EL layer with the above-describedstructure, recombination of carriers occur in the emitting layer,whereby the EL element emits light. Incidentally, “EL element emitslight” is herein called “EL element is driven”.

[0010] As a driving method for the EL display device, there is an activematrix type EL display device.

[0011]FIG. 3 shows an example of the construction of a pixel portion ofan active matrix type EL display device. A gate signal line (G1 to Gy)to which a selection signal is to be inputted from a gate signal linedriver circuit is connected to the gate electrode of a switching TFT 301which is provided in each pixel of the pixel portion. Either one of thesource and drain regions of the switching TFT 301 provided in each pixelis connected to a source signal line (S1 to Sx) to which a signal is tobe inputted from a source signal line driver circuit, while the other isconnected to the gate electrode of an EL driving TFT 302 and to eitherone of the electrodes of a capacitor 303 which is provided in eachpixel. The other electrode of the capacitor 303 is connected to a powersupply line (V1 to Vx). Either one of the source and drain regions ofthe EL driving TFT 302 provided in each pixel is connected to the powersupply line (V1 to Vx), while the other is connected to the otherelectrode of the EL element 304 provided in each pixel.

[0012] The EL element 304 has an anode, a cathode and an EL layerprovided between the anode and the cathode. In the case where the anodeof the EL element 304 is connected to the source region or the drainregion of the EL driving TFT 302, the anode and the cathode of the ELelement 304 become a pixel electrode and a counter electrode,respectively. Contrarily, in the case where the cathode of the ELelement 304 is connected to the source region or the drain region of theEL driving TFT 302, the cathode and the anode of the EL element 304become a pixel electrode and a counter electrode, respectively.

[0013] Incidentally, the potential of the counter electrode is hereincalled “counter potential”, and a power source for applying the counterpotential to the counter electrode is herein called “counter powersource”. The difference between the potential of the pixel electrode andthe potential of the counter electrode is an EL driving voltage, and theEL driving voltage is applied to the EL layer.

[0014] As a gray scale display method for the above-described EL displaydevice, there are an analog gray scale method and a time gray scalemethod.

[0015] First, the analog gray scale method for the EL display devicewill be described below. FIG. 4 is a timing chart showing the case wherethe display device shown in FIG. 3 is driven by the analog gray scalemethod. The period from the moment when one gate signal is selecteduntil the moment when the next gate signal line is selected is hereincalled “one line period (L)”. The period from the moment when one imageis selected until the moment when the next image is selected correspondsto one frame period. In the case of the EL display device shown in FIG.3, since the number of gate signal lines is “y”, y-number of lineperiods (L1 to Ly) are provided in one frame period.

[0016] As the resolution of the EL display device becomes higher, thenumber of line periods for one frame period becomes larger, and thedriver circuit of the EL display device must be driven at a higherfrequency.

[0017] The power source lines (V1 to Vx) are kept at a constant voltage(power source potential). In addition, the counter potential is keptconstant. The counter potential has a potential difference from thepower source potential to such an extent that the EL elements emitlight.

[0018] In the first line period (L1), a selection signal from the gatesignal line driver circuit is inputted to the gate signal line G1. Then,analog video signals are inputted to the source signal lines (S1 to Sx)in this order.

[0019] Since all the switching TFTs 301 connected to the gate signalline G1 are turned on, the analog video signals which have been inputtedto the source signal lines (S1 to Sx) are respectively inputted to theEL driving TFTs 302 via the switching TFTs 301.

[0020] According to the potential of the analog video signal inputted toeach of the pixels when the switching TFT 301 is turned on, the gatevoltage of the EL driving TFT 302 varies. At this time, the draincurrent of the EL driving TFT 302 is determined at a 1-to-1 ratio to thegate voltage thereof in accordance with the Id-Vg characteristic of theEL driving TFT 302. Specifically, according to the potential of theanalog video signals inputted to the gate electrode of the EL drivingTFT 302, the potential of the drain region of the EL driving TFT 302 (anEL driving voltage corresponding to the on state of the switching TFT301) is determined and a predetermined drain current flows into the ELelement 304, and the EL element 304 emits light at the amount ofemission corresponding to the amount of the drain current.

[0021] When the above-described operations are repeated until thetermination of inputting the analog video signals to the respectivesource signal lines (S1 to Sx), the first line period (L1) terminates.Incidentally, one line period may also be defined as the sum of theperiod required until the termination of inputting the analog videosignals to the respective source signal lines (S1 to Sx) and ahorizontal retrace period. Then, the second line period (L2) starts, anda selection signal is inputted to the gate signal line G2. Similarly tothe first line period (L1), analog video signals are inputted to thesource signal lines (S1 to Sx) in this order.

[0022] When selection signals are inputted to all the gate signal lines(G1 to Gy), all the line periods (L1 to Ly) terminate. When all the lineperiods (L1 to Ly) terminate, one frame period terminates. During oneframe period, all the pixels perform displaying and one image is formed.Incidentally, one frame period may also be defined as the sum of all theline periods (L1 to Ly) and a vertical retrace period.

[0023] As described above, the amounts of emissions of the respective ELelements are controlled by the analog video signals, and gray scaledisplay is provided by the control of the amounts of emissions. In thismanner, in the analog gray scale method, gray scale display is carriedout by the variations in the potentials of the respective analog videosignals inputted to the source signal lines.

[0024] The time gray scale method will be described below.

[0025] In the time gray scale method, digital signals are inputted topixels to select the emitting states or the non-emitting states of therespective EL elements, whereby gray scales are represented by thecumulation of periods per frame period during which each of the ELelements.

[0026] In the following description, 2^(n) gray scales (n is a naturalnumber) are represented. FIG. 5 is a timing chart showing the case wherethe display device shown in FIG. 3 is driven by the time gray scalemethod. One frame period is divided into n-number of sub-frame periods(SF₁ to SF_(n)). Incidentally, the period for which all the pixels ofthe pixel portion displays one image is called “one frame period (F)”.Plural periods into which one frame period is divided are called“sub-frame periods”, respectively. As the number of gray scalesincreases, the number by which one frame period is divided alsoincreases, and the driver circuit of the EL display device must bedriven at a higher frequency.

[0027] One sub-frame period is divided into a write period (Ta) and adisplay period (Ts). The write period is the period for which digitalsignals are inputted to all the pixels during one sub-frame period, andthe display period (also called “lighting period”) is the period forwhich the respective EL display devices assume their emitting states ornon-emitting states in accordance with the input digital signals,thereby performing displaying.

[0028] The EL driving voltage shown in FIG. 5 represents the EL drivingvoltage of an EL element for which emitting state is selected.Specifically, the EL driving voltage (FIG. 5) of the EL element forwhich emitting state is selected is 0 V during the write period, andhas, during the display period, a magnitude which enables the EL elementto emit light.

[0029] The counter potential is controlled by an external switch (notshown) so that the counter potential is kept at approximately the samelevel as the power source potential during the write period, and has,during the display period, a potential difference from the power sourcepotential to such an extent that the EL element can emit light.

[0030] The write period and the display period of each sub-frame periodwill first be described in detail with reference to FIGS. 3 and 5, andsubsequently, the time gray scale method will be described.

[0031] First, a gate signal is inputted to the gate signal line G1, andall the switching TFTs 301 connected to the gate signal line G1 areturned on. Then, digital signals are inputted to the source signal lines(S1 to Sx) in that order. The counter potential is kept at the samelevel as the potential of the power supply lines (V1 to Vx) (powersource potential). Each of the digital signals has information of “0” or“1”. Each of the digital signals of “0” or “1” means a signal which hasa voltage of high level or low level.

[0032] Then, the digital signals which have been inputted to the sourcesignal lines (S1 to Sx) are respectively inputted to the gate electrodesof the EL driving TFTs 302 via the switching TFTs 301 which are in theon state. The respective digital signals are also inputted to thecapacitors 303.

[0033] Then, the above-described operations are repeated by inputtinggate signals to the respective gate signal lines (G2 to Gy), wherebydigital signals are inputted to all the pixels and the input digitalsignal is held in each of the pixels. The period required until thedigital signals are inputted to all the pixels is called “write period”.

[0034] When the digital signals are inputted to all the pixels, all theswitching TFTs 301 are turned off. Thus, an external switch (not shown)connected to the counter electrode causes the counter potential to varyso that a potential difference which enables the EL element 304 to emitlight is produced between the counter potential and the power sourcepotential.

[0035] In the case where the digital signals have information of “0”,the EL driving TFTs 302 are turned off and the EL elements 304 do notemit light. Contrarily, in the case where the digital signals haveinformation of “1”, the EL driving TFTs 302 are turned on. Consequently,the pixel electrodes of the respective EL elements 304 are kept atapproximately the same potential as the power source potential, and theEL elements 304 emit light. In this manner, the emitting states or thenon-emitting states of the EL elements 304 are selected in accordancewith the information of the digital signals, and all the pixels performdisplaying at the same time. When all the pixels perform display, animage is formed. The period for which the pixels perform displaying iscalled “display period”.

[0036] The lengths of the write periods (T_(a1) to T_(an)) of all then-number of sub-frame periods (SF₁ to SF_(n)) are the same. The displayperiods (Ts) of the respective sub-frame periods (SF₁ to SF_(n)) aredenoted by T_(s1) to T_(sn).

[0037] The lengths of the respective display periods are set to becomeT_(s1):T_(s2):T_(s3): . . . :T_(s(n−)1):T_(sn)=2⁰:2⁻¹:2²: . . .:2^(−(n−2)):2^(−(n−1)), respectively. By combining the desired ones ofthese display periods, it is possible to provide display in the desirednumber of gray scales within 2^(n) gray scales.

[0038] The display period is any one of T_(s1) to T_(sn). Here, it isassumed that predetermined pixels are turned on for the period ofT_(s1).

[0039] Then, when the next write period starts and data signals areinputted to all the pixels, the next display period starts. At thistime, the display period is any one of T_(s2) to T_(sn). Here, it isassumed that predetermined pixels are turned on for the period ofT_(s2).

[0040] It is assumed that the same operations are repeated as to theremaining (n−2)-number of sub-frames, whereby the display periods areset as T_(s3), T_(s4), . . . , T_(sn) in this order and predeterminedpixels are turned on during each of the sub-frames.

[0041] When the n-number of sub-frame periods appear, one frame periodterminates. At this time, the gray scale of a pixel is determined bycumulatively calculating the length of the display period for which thepixel has been turned on. For example, assuming that n=8 and theobtainable luminance in the case where the pixel emits light for all thedisplay period is 100%, if the pixel emits light during T_(s1) andT_(s2), a luminance of 75% can be represented, and if T_(s3), T_(s5) andT_(s8) are selected, a luminance of 16% can be realized.

[0042] Incidentally, in the driving method using the time gray scalemethod which represents gray scales by inputting n-bit digital signals,the number of plural sub-frame periods into which one frame period isdivided, the lengths of the respective sub-frame periods and the likeare not limited to the above-described examples.

[0043] The above-described analog gray scale method has problems to bedescribed below.

[0044] The analog gray scale method has the problem that the unevennessof the characteristics of TFTs greatly affects gray scale display. Forexample, it is assumed that the Id-Vg characteristics of switching TFTsdiffer between two pixels which represent the same gray scale (thecharacteristic of either one of the pixels is shifted as a whole to aplus or minus side relative to the characteristic of the other).

[0045] In this case, the drain currents of the respective switching TFTstake different values, and gate voltages with different values areapplied to the EL driving TFTs of the respective pixels. In other words,different amounts of currents flow into the EL elements of therespective pixels, and as a result, the amounts of emissions from the ELelements differ from each other and the same gray scale cannot berepresented.

[0046] Even if equal gate voltages are applied to the EL driving TFTs ofthe respective pixels, the EL driving TFTs cannot output the same amountof drain current so long as the Id-Vg characteristics of the EL drivingTFTs are not even. For this reason, if the Id-Vg characteristics of theswitching TFTs slightly differ from each other, the amounts of currentsoutputted from the EL driving TFTs greatly differ from each other evenwhen equal gate voltages are applied to the EL driving TFTs. As aresult, owing to a slight unevenness of the Id-Vg characteristics, theamounts of emissions from the EL elements greatly differ betweenadjacent pixels even if signals of the same voltage are applied to theEL driving TFTs.

[0047] Gray scale display actually becomes far more non-uniform owing toa synergistic effect of the unevenness of the characteristics of theswitching TFTs and the unevenness of the characteristics of the ELdriving TFTs. Thus, analog gray scale display is extremely sensitive tothe unevenness of the characteristics of TFTs. Accordingly, when this ELdisplay device provides gray scale display, there is the problem thatthe display becomes considerably uneven.

[0048] The time gray scale method has a problem to be described below.

[0049] In the time gray scale method, the luminance of an EL element isrepresented by the time for which a current flows in the EL element andthe EL element emits light. Accordingly, it is possible to greatlysuppress the non-uniformity of display due to the unevenness of thecharacteristics of TFTs, which is a problem in the analog gray scalemethod. However, there is another problem.

[0050] The current which flows in the EL element is controlled by avoltage to be applied across both electrodes of the EL element (ELdriving voltage). This EL driving voltage is a voltage obtained bysubtracting the voltage across the drain and the source of an EL drivingTFT from the potential difference between a power source potential and acounter potential. In order to avoid the influence of the non-uniformityof drain-source voltages due to the unevenness of the characteristics ofEL driving TFTs and keep the EL driving voltage constant, the voltageacross the drain and the source of the EL driving TFT is set to be farsmaller than the EL driving voltage. At this time, the EL driving TFT isoperating in a linear region.

[0051] In the TFT operation, the linear region corresponds to theoperating region in which a voltage V_(DS) across the drain and thesource of the TFT is smaller than a gate voltage V_(GS) of the TFT.

[0052] Here, the current flowing between both electrodes of the ELelement is influenced by temperature. FIG. 17 is a graph showing thetemperature characteristic of the EL element. From this graph, it ispossible to know the amounts of currents which flow between bothelectrodes of the EL element with respect to voltages applied acrossboth electrodes of the EL element at certain temperatures. A temperatureT₁ is higher than a temperature T₂, and the temperature T₂ is higherthan a temperature T₃. As can be seen from FIG. 17, even if the voltageapplied across the both electrodes of the EL element in the pixelportion is the same, the current flowing between both electrodes of theEL element becomes larger owing to the temperature characteristic of theEL element as the temperature of the EL element becomes higher.

[0053] The luminance of the EL element is proportional to the amount ofcurrent flowing between both electrodes of the EL element.

[0054] In this manner, the time gray scale method has the problem thatthe current flowing between both electrodes of the EL element variesowing to variations in the environmental temperature at which the ELdisplay device is used if a constant voltage is continuously appliedacross both electrodes of the EL element, and the luminance of the ELdisplay device varies and accurate gray scale display becomesimpossible.

[0055] In the active matrix type EL display device, for theabove-described reasons, if the conventional analog gray scale method ortime gray scale method is used, it is impossible to perform accurategray scale display.

SUMMARY OF THE INVENTION

[0056] The invention provides a method of driving an EL display device,which enables accurate gray scale display and hence high-quality imagedisplay.

[0057] In accordance with the present invention, an active matrix typeEL display device is driven by a time gray scale method. At this time,an EL driving TFT is operated in a saturation region to keep its draincurrent constant with respect to temperature variations.

[0058] Accordingly, it is possible to keep constant a current whichflows between both electrodes of an EL element, with respect to theunevenness of the characteristics of TFTs and variations inenvironmental temperature, whereby it is possible to provide a method ofdriving an EL display device which method enables accurate gray scaledisplay and hence high-quality image display.

[0059] The construction of the invention will be described below.

[0060] In accordance with the present invention, there is provided amethod of driving a display device which includes pixels each having anEL element and a transistor, and the method includes the step ofdividing one frame period into plural sub-frame periods and applying afirst gate voltage or a second gate voltage to a gate electrode of thetransistor during each of the plural sub-frame periods. When the firstgate voltage is applied to the gate electrode of the transistor, a draincurrent of the transistor flows across both electrodes of the EL elementand the EL element is placed into an emitting state, and when the secondgate voltage is applied to the gate electrode of the transistor, thetransistor is placed into a non-conductive state and the EL element isplaced into a non-emitting state. An absolute value of the first gatevoltage is not greater than an absolute value of a voltage across adrain and a source of the transistor.

[0061] In accordance with the present invention, there is provided amethod of driving a display device which includes pixels each having anEL element, a transistor and a resistor, and the method includes thestep of dividing one frame period into plural sub-frame periods andapplying a first gate voltage or a second gate voltage to a gateelectrode of the transistor during each of the plural sub-frame periods.When the first gate voltage is applied to the gate electrode of thetransistor, a drain current of the transistor flows across the resistorand both electrodes of the EL element and the EL element is placed intoan emitting state, and when the second gate voltage is applied to thegate electrode of the transistor, the transistor is placed into anon-conductive state and the EL element is placed into a non-emittingstate. An absolute value of the first gate voltage is not greater thanan absolute value of a voltage across a drain and a source of thetransistor.

[0062] The method of driving a display device may be a method in whichas the ratio of a gate width to a gate length of the transistor issmaller than 1, the absolute value of the first gate voltage applied tothe gate electrode of the transistor is larger without exceeding theabsolute value of the voltage across the drain and the source of thetransistor.

[0063] The method of driving a display device may be a method in whichthe EL element enables color display by using an EL layer which emitslight of one color, in combination with a color conversion layer.

[0064] The method of driving a display device may be a method in whichthe EL element enables color display by using an EL layer which emitswhite light, in combination with a color filter.

[0065] The method of driving a display device may be a method in whichthe EL layer of the EL element is made of a low molecular weight organicmaterial or a polymeric organic material.

[0066] The method of driving a display device may be a method in whichthe low molecular weight organic material is Alq₃(tris-8-quinolinolato-aluminum) or TPD (triphenylamine derivative) Themethod of driving a display device may be a method in which thepolymeric organic material is PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole) or polycarbonate.

[0067] The method of driving a display device may be a method in whichthe EL layer of the EL element is an inorganic material.

[0068] The method of driving a display device may be used in a videocamera, an image reproducing apparatus, a head-mounted display, a mobiletelephone or a mobile information terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0069]FIGS. 1A and 1B are views showing a method of driving a displaydevice according to the invention;

[0070]FIG. 2 is a view showing the construction of a pixel portion of adisplay device using the driving method according to the invention;

[0071]FIG. 3 is a view showing the construction of a pixel portion of anEL display device;

[0072]FIG. 4 is a timing chart showing a method of driving a related artEL display device;

[0073]FIG. 5 is a timing chart showing a method of driving an EL displaydevice;

[0074]FIG. 6 is a circuit diagram showing a source signal line drivercircuit of the EL display device;

[0075]FIG. 7 is a top plan view of a latch of the EL display device;

[0076]FIGS. 8A to 8C are views showing the process of fabricating an ELdisplay device;

[0077]FIGS. 9A, 9B and 9C are views showing the process of fabricatingthe EL display device;

[0078]FIGS. 10A and 10B are views showing the process of fabricating theEL display device;

[0079]FIGS. 11A and 11B are a top plan view and a cross-sectional viewof an EL display device;

[0080]FIGS. 12A and 12B are a top plan view and a cross-sectional viewof an EL display device;

[0081]FIG. 13 is a cross-sectional view of a pixel portion of an ELdisplay device;

[0082]FIG. 14 is a cross-sectional view of a pixel portion of an ELdisplay device;

[0083]FIGS. 15A and 15B are a top plan view and a cross-sectional viewof an EL display device;

[0084]FIG. 16 is a cross-sectional view of an EL display device;

[0085]FIG. 17 is a graph showing the temperature characteristic of an ELelement; and

[0086]FIGS. 18A to 18E are views showing examples of electronicequipment provided with EL display devices using driving methodsaccording to the invention.

DESCRIPTION OF THE PREFFERED EMBODIMENTS

[0087] (Embodiment Mode)

[0088] An embodiment mode of the present invention will be describedbelow in detail with reference to FIGS. 1A and 1B.

[0089]FIG. 1A is a circuit diagram showing the construction of a pixelof an EL display device according to the present invention. The gateelectrode of a switching TFT 903 is connected to a gate signal line 906.Either one of the source and drain regions of the switching TFT 903 isconnected to a source signal line 905, while the other is connected tothe gate electrode of an EL driving TFT 900 and to a capacitor 904.Either one of the source and drain regions of the EL driving TFT 900 isconnected to a power supply line 902, while the other is connected tothe anode or the cathode of the EL element 901.

[0090] Let V_(GS) represent a voltage (gate voltage) applied across thegate and the source of the EL driving TFT 900 from the switching TFT903. Let V_(DS) represent a voltage (drain-source voltage) appliedacross the drain and the source of the EL driving TFT 900, and let I_(D)represent a current (drain current) which flows between the drain andthe source at this time. This drain current I_(D) is inputted to the ELelement 901. Letting V_(EL) represent a voltage (EL driving voltage)applied across both electrodes of the EL element 901, a voltage VINapplied across a pixel portion (the counter electrode of the EL element901) and a power supply line 902 is given as the sum of the drain-sourcevoltage V_(DS) and the EL driving voltage V_(EL).

[0091]FIG. 1B is a graph showing the relationship between thedrain-source voltage V_(DS) and drain current I_(D). The gate voltageV_(GS) is constant. In this graph, the region in which the drain currentI_(D) has a one to one correspondence to the drain-source voltage V_(DS)is called a linear region, which corresponds to the case in which thedrain-source voltage V_(DS) is small compared to the gate voltageV_(GS). The region in which the drain current I_(D) is approximatelyconstant with respect to the drain-source voltage V_(DS) is called asaturation region, which corresponds to the case in which thedrain-source voltage V_(DS) is greater than or equal to the gate voltageV_(GS).

[0092] In the method of driving the EL display device with theconventional time gray scale method, control is executed so that thevoltage applied across both electrodes of the EL element 901 is madeconstant. In this case, if the drain-source voltage V_(DS) of the ELdriving TFT 900 fluctuates owing to the unevenness of the characteristicof the TFT 900, the EL driving voltage V_(EL) will be influenced. Forthis reason, in order to suppress the influence of such unevenness asgreatly as possible, the drain-source voltage V_(DS) of the EL drivingTFT 900 is set smaller than the EL driving voltage V_(EL) SO that amajor part of the voltage VIN inputted to the pixel can be appliedacross both electrodes of the EL element 901. Accordingly, the ELdriving TFT 900 is made to operate in the linear region whichcorresponds to the case in which the drain-source voltage V_(DS) issmall compared to the gate voltage V_(GS).

[0093] In the EL display device according to the present invention, thedrain-source voltage V_(DS) of the EL driving TFT 900 is set to the gatevoltage V_(GS) or more, and the EL driving TFT 900 is made to operate inthe saturation region in which the constant drain current I_(D) flowsirrespective of the drain-source voltage V_(DS). Accordingly, a constantcurrent is consistently supplied to the EL element 901 irrespective oftemperature changes.

[0094] Numerical examples of the voltages applied to the EL element 901and the EL driving TFT 900 are as follows.

[0095] For example, the threshold voltage of the EL driving TFT 900 ismade approximately 2V. In the case where the gate voltage V_(GS) of theEL driving TFT 900 is made 5 V with the emitting state of the EL element901 of the pixel being selected, the voltage between the counterelectrode of the EL element 901 and the power supply line 902 (thedifference between the counter potential and the power source potential)during the display period is made approximately 15 V. At this time, thevoltage V_(EL) across both electrodes of the EL element 901 takes avalue of approximately 5-10 V, and the drain-source voltage V_(DS) ofthe EL driving TFT 900 becomes approximately 5 V or more. At this time,the drain-source voltage V_(DS) of the EL driving TFT 900 becomes thegate voltage V_(GS) or more, and the EL driving TFT 900 operates in thesaturation region.

[0096] In this manner, a constant current consistently flows in the ELelement 901 irrespective of temperature changes, whereby the EL element901 emits light at a constant luminance.

[0097] [Embodiments]

[0098] Embodiments of the invention will be described below.

[0099] (Embodiment 1)

[0100] Embodiment 1 relates to the method in the above description ofthe embodiment mode of the present invention, i.e., the method ofoperating the EL driving TFT in the saturation region to keep constantthe drain current I_(D) which flows across both electrodes of the ELelement, and the following description of Embodiment 1 is a method ofsuppressing the influence of the unevenness of the characteristics of ELdriving TFTs. The following description uses the same reference numeralsand used in FIG. 1A as well as newly added ones.

[0101] In the case where the EL driving TFT 900 is operated in thesaturation region, the following equation (1) is obtained:

I _(D)=α(W/L)(V _(GS) −V _(th))²  (1)

[0102] In equation (1), I_(D) is the drain current, V_(GS) is the gatevoltage, V_(th) is the threshold voltage, W is the gate width, L is thegate length, and α is a constant. In this case, since the thresholdvoltage V_(th) has variations, the drain current I_(D) also hasvariations.

[0103] To suppress these variations, the W/L ratio of the gate width Wto the gate length L is made small, while the gate voltage V_(GS) ismade large, within the range in which the EL driving TFT 900 operates inthe saturation region. In this manner, it is possible to suppress thevariations of the drain current I_(D) due to the variations of thethreshold voltage V_(th) of the EL driving TFT 900.

[0104] For example, it is assumed that the threshold voltage V_(th)takes a value of 2±0.1 V and has a 5% variation, and that the gatevoltage V_(GS) is 3 V when W/L is 8. When the value of the drain currentI_(D) at this time is calculated, the resultant value has an about 20%variation.

[0105] Here, let I_(O) be the average value of the drain current I_(D)If W/L is made 0.5, the gate voltage V_(GS) needs to be made about 6 Vso that the average value I_(O) of the drain current I_(D) is made thesame as when W/L is 8. According to the calculation of the value of thedrain current I_(D) for the gate voltage V_(GS) of 6 V, it is possibleto suppress the variation of the value to an about 5% variation.

[0106] In this manner, it is desirable to make the value of W/L lessthan 1, preferably 0.5 or less.

[0107] (Embodiment 2)

[0108] Embodiment 2 relates to the method in the above description theembodiment mode of the present invention, i.e., the method of operatingthe EL driving TFT in the saturation region to keep constant the draincurrent I_(D) which flows across both electrodes of the EL element, andthe following description of Embodiment 2 is a method of suppressing theinfluence of the unevenness of the characteristics of EL driving TFTs bya method different from that used in Embodiment 1.

[0109]FIG. 2 is a circuit diagram showing the construction of a pixelportion of an EL display device according to Embodiment 2. The pixelportion shown in FIG. 2 is the same in basic structure as that shown inFIG. 1A, and in the following description, the modified portions of theconstruction shown in FIG. 1A are denoted by different referencenumerals.

[0110] The gate electrode of the switching TFT 903 is connected to thegate signal line 906. Either one of the source and drain regions of theswitching TFT 903 is connected to the source signal line 905, while theother is connected to the gate electrode of the EL driving TFT 900 andto either one of the electrodes of the capacitor 904. The otherelectrode of the capacitor 904 is connected to the power supply line902. Either one of the source region and the drain region of the ELdriving TFT 900 is connected to the power supply line 902 via a resistor907, while the other is connected to the anode or the cathode of the ELelement 901.

[0111] In the case of the construction of the pixel according toEmbodiment 2, the equation (1) shown in Embodiment 1 and the followingequation (2) are satisfied at the same time:

V=V _(GS) +RI _(D)  (2)

[0112] In equation (2), V is a potential difference given between thegate electrode of the EL driving TFT 900 and the power supply line 902,and R is the resistance value of the resistor 907.

[0113] The gate voltage V_(GS) and the drain current I_(D), in the casewhere the resistor 907 is disposed as shown in FIG. 2, are found fromequations (1) and (2). At this time, the variation of the drain currentI_(D) relative to the variation of the threshold voltage V_(th) iscalculated.

[0114] For example, in equations (1) and (2), it is assumed that cc is2×10⁻⁶F/V·s and W/L is 1, and that V_(th) takes a value of 2±0.1 V andhas a 5% variation.

[0115] First, consideration is given to the case where R=0 (the resistor907 is absent). If V is 4 V, the gate voltage V_(GS) coincides with V at4 V. The variation of the drain current I_(D) at this time is about 10%.At this time, the average value of the drain current I_(D) is about8×10⁻⁶ A.

[0116] Then, consideration is given to the case where R=1×10⁶ Ω. To holdthe average value of the drain current I_(D) at about 8×10⁻⁶ A, V ismade 12 V. At this time, the variation of the drain current I_(D)relative to the variation of the threshold voltage V_(th) is suppressedto about 1%.

[0117] Then, consideration is given to the case where R=2×10⁶ Ω. To holdthe average value of the drain current I_(D) at about 8×10⁻⁶ A, V ismade 20 V. At this time, the variation of the drain current I_(D)relative to the variation of the threshold voltage V_(th) is suppressedto about 0.6%.

[0118] In this manner, with disposing the resistor 907 and setting itsresistance value large, it is possible to suppress the variation of thedrain current I_(D) relative to the variation of the threshold voltageV_(th).

[0119] Embodiment 2 can be freely carried out in combination withEmbodiment 1.

[0120] (Embodiment 3)

[0121] Note that a description is set forth regarding a step forfabricating TFTs for driver circuit (a source signal line driver circuitand a gate signal line driver circuit) provided in the pixel portion ofa display device using the driver method of the present invention andperiphery portion of the pixel portion. For the simplicity of theexplanation, a CMOS circuit is shown in figures, which is a fundamentalstructure circuit for the driver circuit portion.

[0122] First, as shown in FIG. 8A, a base film 5002 made of aninsulating film such as a silicon oxide film, a silicon nitride film, ora silicon oxynitride film, is formed on a substrate 5001 made of a glasssuch as barium borosilicate glass or aluminum borosilicate glass,typically a glass such as Corning Corp. #7059 glass or #1737 glass. Forexample, a lamination film of a silicon oxynitride film 5002 a,manufactured from SiH₄, NH₃, and N₂O by plasma CVD, and formed having athickness of 10 to 200 nm (preferably between 50 and 100 nm), and ahydrogenated silicon oxynitride film 5002 b, similarly manufactured fromSiH₄ and N₂O, and formed having a thickness of 50 to 200 nm (preferablybetween 100 and 150 nm), is formed. A two layer structure is shown forthe base film 5002 in Embodiment 3, but a single layer film of theinsulating film, and a structure in which more than two layers arelaminated, may also be formed.

[0123] Island shape semiconductor layers 5003 to 5006 are formed bycrystalline semiconductor films made from a semiconductor film having anamorphous structure, using a laser crystallization method or a knownthermal crystallization method. The thickness of the island shapesemiconductor layers 5003 to 5006 may be formed from 25 to 80 nm(preferably between 30 and 60 nm). There are no limitations placed onthe materials for forming a crystalline semiconductor film, but it ispreferable to form the crystalline semiconductor films by silicon or asilicon germanium (SiGe) alloy.

[0124] A laser such as a pulse oscillation type or continuous lightemission type excimer laser, a YAG laser, or a YVO₄ laser can be used tofabricate the crystalline semiconductor films by the lasercrystallization method. A method of condensing laser light emitted froma laser oscillator into a linear shape by an optical system and thenirradiating the light to the semiconductor film may be used when thesetypes of lasers are used. The crystallization conditions may be suitablyselected by the operator, but when using the excimer laser, the pulseoscillation frequency is set to 30 Hz, and the laser energy density isset form 100 to 400 mJ/cm² (typically between 200 and 300 mJ/cm²).Further, when using the YAG laser, the second harmonic is used and thepulse oscillation frequency is set from 1 to 10 kHz, and the laserenergy density may be set from 300 to 600 mJ/cm² (typically between 350and 500 mJ/cm²). The laser light condensed into a linear shape with awidth of 100 to 1000 μm, for example 400 μm, is then irradiated over theentire surface of the substrate. This is performed with an overlap ratioof 80 to 98% for the linear laser light.

[0125] A gate insulating film 5007 is formed covering the island shapesemiconductor layers 5003 to 5006. The gate insulating film 5007 isformed of an insulating film containing silicon with a thickness of 40to 150 nm by plasma CVD or sputtering. A 120 nm thick silicon oxynitridefilm is formed in Embodiment 3. The gate insulating film is not limitedto this type of silicon oxynitride film, of course, and other insulatingfilms containing silicon may also be used in a single layer or in alamination structure. For example, when using a silicon oxide film, itcan be formed by plasma CVD with a mixture of TEOS (tetraethylorthosilicate) and O₂, at a reaction pressure of 40 Pa, with thesubstrate temperature set from 300 to 400° C., and by discharging at ahigh frequency (13.56 MHZ) electric power density of 0.5 to 0.8 W/cm².Good characteristics as a gate insulating film can be obtained bysubsequently performing thermal annealing, at between 400 and 500° C.,of the silicon oxide film thus manufactured.

[0126] A first conductive film 5008 and a second conductive film 5009are then formed on the gate insulating film 5007 in order to form gateelectrodes. The first conductive film 5008 is formed of a Ta film with athickness of 50 to 100 nm, and the second conductive film 5009 is formedof a W film having a thickness of 100 to 300 nm, in Embodiment 3.

[0127] The Ta film is formed by sputtering, and sputtering of a Tatarget is performed by Ar. If appropriate amounts of Xe and Kr are addedto Ar, the internal stress of the Ta film is relaxed, and film peelingcan be prevented. The resistivity of an α phase Ta film is about 20μΩcm, and it can be used in the gate electrode, but the resistivity of aβ phase Ta film is about 180 μΩcm and it is unsuitable for the gateelectrode. The α phase Ta film can easily be obtained if a tantalumnitride film, which possesses a crystal structure similar to that of αphase Ta, is formed with a thickness of about 10 to 50 nm as a base fora Ta film in order to form the α phase Ta film.

[0128] The W film is formed by sputtering with a W target, which canalso be formed by thermal CVD using tungsten hexafluoride (WF₆).Whichever is used, it is necessary to make the film become lowresistance in order to use it as the gate electrode, and it ispreferable that the resistivity of the W film be made equal to or lessthan 20 μΩcm. The resistivity can be lowered by enlarging the crystalgrains of the W film, but for cases in which there are many impurityelements such as oxygen within the W film, crystallization is inhibited,thereby the film becomes high resistance. A W target having a purity of99.9999% is thus used in sputtering. In addition, by forming the W filmwhile taking sufficient care that no impurities from the gas phase areintroduced at the time of film formation, the resistivity of 9 to 20μΩcm can be achieved.

[0129] Note that, although the first conductive film 5008 is a Ta filmand the second conductive film 5009 is a W film in Embodiment 3, bothmay also be formed from an element selected from the group consisting ofTa, W, Ti, Mo, Al, and Cu, or from an alloy material having one of theseelements as its main constituent, and a chemical compound material.Further, a semiconductor film, typically a polycrystalline silicon filminto which an impurity element such as phosphorus is doped, may also beused. Examples of preferable combinations other than that used inEmbodiment 3 include: forming the first conductive film 5008 by tantalumnitride (TaN) and combining it with the second conductive film 5009formed from a W film; forming the first conductive film 5008 by tantalumnitride (TaN) and combining it with the second conductive film 5009formed from an Al film; and forming the first conductive film 5008 bytantalum nitride (TaN) and combining it with the second conductive film5009 formed from a Cu film.

[0130] Then, mask 5010 are formed from resist, and a first etchingtreatment is performed in order to form electrodes and wirings. An ICP(inductively coupled plasma) etching method is used in Embodiment 3. Agas mixture of CF₄ and Cl₂ is used as an etching gas, and a plasma isgenerated by applying a 500 W RF electric power (13.56 MHZ) to a coilshape electrode at 1 Pa. A 100 W RF electric power (13.56 MHZ) is alsoapplied to the substrate side (test piece stage), effectively applying anegative self-bias voltage. In case of mixing CF₄ and Cl₂, the W filmand the Ta film are etched to the approximately same level.

[0131] Edge portions of the first conductive layer and the secondconductive layer are made into a tapered shape in accordance with theeffect of the bias voltage applied to the substrate side under the aboveetching conditions by using a suitable resist mask shape. The angle ofthe tapered portions is from 15 to 45°. The etching time may beincreased by approximately 10 to 20% in order to perform etching withoutany residue remaining on the gate insulating film. The selectivity of asilicon oxynitride film with respect to a W film is from 2 to 4(typically 3), and therefore approximately 20 to 50 nm of the exposedsurface of the silicon oxynitride film is etched by this over-etchingprocess. First shape conductive layers 5011 to 5016 (first conductivelayers 5011 a to 5016 a and second conductive layers 5011 b to 5016 b)are thus formed of the first conductive layers and the second conductivelayers in accordance with the first etching process. Reference numeral5007 denotes a gate insulating film, and the regions not covered by thefirst shape conductive layers 5011 to 5016 are made thinner by etchingof about 20 to 50 nm.

[0132] A first doping process is then performed, and an impurity elementwhich imparts n-type conductivity is added. Ion doping or ion injectionmay be performed for the method of doping. Ion doping is performed underthe conditions of a dose amount of from 1×10¹³ to 5×10¹⁴ atoms/cm² andan acceleration voltage of 60 to 100 keV. A periodic table group 15element, typically phosphorus (P) or arsenic (As) is used as theimpurity element which imparts n-type conductivity, and phosphorus (P)is used here. The conductive layers 5011 to 5015 become masks withrespect to the n-type conductivity imparting impurity element in thiscase, and first impurity regions 5017 to 5025 are formed in aself-aligning manner. The impurity element which imparts n-typeconductivity is added to the first impurity regions 5017 to 5025 with aconcentration in the range of 1×10²⁰ to 1×10²¹ atoms/cm³. (FIG. 8B)

[0133] A second etching process is performed next without removing aresist mask, as shown in FIG. 8C. The W film is etched selectively usinga mixture of CF₄, Cl₂, and O₂ as a etching gas. The second shapeconductive layers 5026 to 5031 (first conductive layers 5026 a to 5031 aand second conductive layers 5026 b to 5031 b) are formed by secondetching process. Reference numeral 5007 denotes a gate insulating film,and regions not covered by the second shape conductive layers 5026 to5031 are additionally etched on the order of 20 to 50 nm, formingthinner regions.

[0134] The etching reaction of a W film or a Ta film in accordance witha mixed gas of CF₄ and Cl₂ can be estimated from the radicals generatedand from the ion types and vapor pressures of the reaction products.Comparing the vapor pressures of fluorides and chlorides of W and Ta,the W fluoride compound WF₆ is extremely high, and the vapor pressuresof WCl₅, TaF₅, and TaCl₅ are of similar order. Therefore the W film andthe Ta film are both etched by the CF₄ and Cl₂ gas mixture. However, ifa suitable quantity of O₂ is added to this gas mixture, CF₄ and O₂react, forming CO and F, and a large amount of F radicals or F ions isgenerated. As a result, the etching speed of the W film having a highfluoride vapor pressure is increased. On the other hand, even if Fincreases, the etching speed of Ta does not relatively increase.Further, Ta is easily oxidized compared to W, and therefore the surfaceof Ta is oxidized by the addition of O₂. The etching speed of the Tafilm is further reduced because Ta oxides do not react with fluorine andchlorine. Therefore, it becomes possible to have a difference in etchingspeeds between the W film and the Ta film, and it becomes possible tomake the etching speed of the W film larger than that of the Ta film.

[0135] A second doping process is then performed, as shown in FIG. 9A.The dose amount is smaller than that of the first doping process in thiscase, and an impurity element which imparts n-type conductivity is dopedunder high acceleration voltage conditions. For example, dopingperformed with the acceleration voltage set from 70 to 120 keV, and adose amount of 1×10¹³ atoms/cm³, and a new impurity region is formedinside the first impurity region is formed inside the first impurityregion formed in the island shape semiconductor layers of FIG. 8B. Thesecond conductive layers 5026 to 5030 are used as masks with respect tothe impurity element, and doping is performed so as to also add theimpurity element into regions under the first conductive layers 5026 ato 5030 a. A concentration of phosphorus (P) added to third impurityregions 5032 to 5036 is provided with a gradual concentration gradientin accordance with a film thickness of the taper portion of the firstconductive layers 5026 a to 5030 a. Further, in the semiconductor layeroverlapping the taper portion of the first conductive layers 5026 a to5030 a, from an end portion of the taper portion of the secondconductive layer toward an inner side, the impurity concentration ismore or less reduced, however, the concentration stays to besubstantially the same degree.

[0136] A third etching process is carried out as shown in FIG. 9B. Thethird etching is carried out by using CHF₆ for an etching gas and usinga reactive ion etching process (RIE process). The third etching processis carried out for partially etching a taper portion of the firstconductive layers 5026 a to 5031 a and reducing a region overlapping thesemiconductor layer. By the third etching, there are formed thirdconductive layers 5037 through 5042 (first conductive layers 5037 a to5042 a and second conductive layers 5037 b to 5042 b). Reference numeral5007 denotes a gate insulating film, and regions not covered by thethird shape conductive layers 5037 to 5042 are additionally etched onthe order of 20 to 50 nm, forming thinner regions.

[0137] By the third etching, there are formed third impurity regions5032 a to 5036 a overlapping the first conductive layers 5037 a to 5041a in third impurity regions 5032 to 5036. Second impurity regions 5032 bto 5036 b between first impurity region and third impurity region.

[0138] Fourth impurity regions 5043 to 5054 added with an impurityelement having a conductivity type which is the opposite of the firstconductivity type impurity element, are then formed as shown in FIG. 9Cin the island shape semiconductor layers 5004, 5006 which form p-channelTFTs. The third shaped conductive layers 5038 b to 5041 b is used as amask with respect to the impurity element, and the impurity regions areformed in a self-aligning manner. The island shape semiconductor layers5003, 5005 and wiring portion 5042 which form n-channel TFTs, arecovered over their entire surface areas by resist mask 5200. Phosphorusis added to the impurity regions 5043 to 5054 at a differentconcentration, and ion doping is performed here using diborane (B₂H₆),so that the respective impurity regions have the impurity concentrationof 2×10²⁰ to 2×10²¹ atoms/cm³.

[0139] Impurity regions are formed in the respective island shapesemiconductor layers by the above processes. The third shaped conductivelayers 5037 to 5041 overlapping the island shape semiconductor layersfunction as gate electrodes. The reference numeral 5042 functions as anisland shape source signal line.

[0140] A process of activating the impurity elements added to therespective island shape semiconductor layers is then performed with theaim of controlling conductivity type after removing the resist mask5200. Thermal annealing using an annealing furnace is performed for thisprocess. In addition, laser annealing and rapid thermal annealing (RTA)can also be applied. Thermal annealing is performed with an oxygenconcentration equal to or less than 1 ppm, preferably equal to or lessthan 0.1 ppm, in a nitrogen atmosphere at 400 to 700° C., typicallybetween 500 and 600° C. Heat treatment is performed for 4 hours at 500°C. in Embodiment 3. However, for cases in which the wiring material usedin the third conductive layers 5037 to 5042 is weak with respect toheat, it is preferable to perform activation after forming an interlayerinsulating film (having silicon as its main constituent) in order toprotect the wirings and the like.

[0141] In addition, heat treatment is performed for 1 to 12 hours at 300to 450° C. in an atmosphere containing between 3 and 100% hydrogen,performing hydrogenation of the island shape semiconductor layers. Thisprocess is one of terminating dangling bonds in the island shapesemiconductor layers by hydrogen which is thermally excited. Plasmahydrogenation (using hydrogen excited by a plasma) may also be performedas another means of hydrogenation.

[0142] As shown in FIG. 10A, a first interlayer insulating film 5055 isformed next of a silicon oxynitride film having a thickness of 100 to200 nm. A second interlayer insulating film 5056 made of an organicinsulating material is then formed on the first interlayer insulatingfilm 5055. After that, the first interlayer film, the second interlayer5056 and the contact hole for the gate insulating film 5007 are formed.The pixel electrode 5063 which is contact to the connect wiring 5062 ispatterned to formed after forming each wirings (including connect wiringand signal wiring) 5057 to 5062 and 5064.

[0143] As the second interlayer insulating film 5056, a film made oforganic resin is used, and as the organic resin, polyimide, polyamide,acrylic, BCB (benzocyclobutene) or the like can be used. Especially,since the second interlayer insulating film 5056 has rather the meaningof flattening, acrylic excellent in flatness is desirable. In thisembodiment, an acrylic film is formed to such a thickness that steppedportions formed by the TFTs can be adequately flattened. It isappropriate that the thickness is preferably made 1 to 5 μm (mostpreferably 2 to 4 μm).

[0144] The formation of the contact holes are performed by dry etchingor wet etching. Contact holes reaching the n-type impurity regions 5017,5018, 5021 and 5023 or the p-type impurity regions 5043 to 5054, acontact hole reaching to a wiring 5042, a contact hole reaching electriccurrent supply line (not shown), and a contact hole (not shown) reachinga gate electrode are formed, respectively.

[0145] Besides, as the wirings (including connect wiring and signalwiring) 5057 to 5062, and 5064, a lamination film of three-layerstructure is used, in which a Ti film with a thickness of 100 nm, analuminum film containing Ti with a thickness of 300 nm, and a Ti filmwith a thickness of 150 nm are continuously formed by sputtering intoone is patterned into a desired shape. Of course, the other conductivefilm may be used.

[0146] Further, in Embodiment 3, an ITO film with a thickness of 110 nmis formed as a pixel electrode 5063, and then subjected to patterning. Acontact is obtained by arranging the pixel electrode 5063 so as tooverlap with the connect wiring 5062 while contacting therewith.Besides, a transparent conductive film in which 2 to 20% of zinc oxideis mixed with indium oxide may be used. This pixel electrode 5063becomes an anode of an EL element (FIG. 10A).

[0147] Then, as shown in FIG. 10B, an insulating film containing silicon(silicon oxide film in Embodiment 3) is formed into a thickness of 500nm, and an opening is formed at a position corresponding to the pixelelectrode 5063 to form the third interlayer insulating film 5065. Uponthe formation of the opening, taper-shape side walls can easily beformed by using a wet etching method. If the side walls of the openingis sufficiently smooth, degradation of the EL layer caused by the stepbecomes a remarkable problem.

[0148] Then, an EL layer 5066 and a cathode (MgAg electrode) 5067 arecontinuously formed by vapor deposition without exposing them to theatmosphere. Note that the thickness of the EL layer 5066 is preferablyset as 80 to 200 nm (typically 100 to 120 nm), and the thickness of thecathode 5067 is preferably set as 180 to 300 nm (typically 200 to 250nm).

[0149] In this step, the EL layer and the cathode are sequentiallyformed with respect to the pixels corresponding to a red color, a greencolor, and a blue color, respectively. Note that, the EL layer lackswithstand property against solutions, and therefore the respectivecolors must be formed individually without using a photolithographytechnology. For that reason, it is preferred that portions other thandesired pixels are masked using metallic masks, and the EL layer and thecathode are selectively formed only for the necessary portions.

[0150] In other words, a mask for masking all the portions except thepixels corresponding to a red color is first set, and the EL layeremitting a red color and the cathode are selectively formed using themask. Then, a mask for masking all the portions except the pixelscorresponding to a green color is set, and the EL layer emitting a greencolor and the cathode are selectively formed using the mask.Succeedingly, similarly, a mask for masking all the portions except thepixels corresponding to a blue color is set, and the EL layer emitting ablue color and the cathode are selectively formed using the mask. Notethat, in this case, a description is made such that a different mask isused for each case, however, the same mask may be used for all thecases.

[0151] Employed in this case is a system in which three kinds of ELelements corresponding to RGB are formed. However, the following systemsmay be used: a system in which an EL element emitting a white color anda color filter are combined; a system in which an EL element emitting ablue or blue-green color and a fluorescing body (fluorescing colorconversion layer: CCM) are combined; and a system in which a transparentelectrode is used for a cathode (opposing electrode) and an EL elementcorresponding to the RGB is overlapped therewith.

[0152] Note that known materials may be used for the EL layer 5066. Asthe known materials, organic materials are preferably used when taking adriver voltage into an account. For example, a four-layer structureconsisting of a hole injection layer, a hole transport layer, a lightemitting layer, and an electron injection layer may be used as the ELlayer.

[0153] Next, the cathode 5067 is formed using a metal mask on the pixelshaving the switching TFTs of which the gate electrodes are connected tothe same gate signal line (pixels on the same line). Note that, inEmbodiment 3, although MgAg is used as the cathode 5067, the presentinvention is not limited to this. Other known materials may be used forthe cathode 5067.

[0154] Finally, a passivation film 5068 made from a silicon nitride filmis formed into a thickness of 300 nm. By forming the passivation film5068, the EL layer 5066 can be protected from moisture, etc., and thereliability of the EL element may be enhanced.

[0155] Consequently, the EL display device with the structure as shownin FIG. 1OB is completed. Note that, in the manufacturing process of theEL display in Embodiment 3, the source signal lines are formed from Taand W, which are materials for forming gate electrodes, and the gatesignal lines are formed from Al, which is a wiring material for formingdrain/source electrode, but different materials may be used.

[0156] Incidentally, the EL display device in Embodiment 3 exhibits thevery high reliability and has the improved operational characteristic byproviding TFTs having the most suitable structure in not only the pixelportion but also the driver circuit portion. Further, it is alsopossible to add a metallic catalyst such as Ni in the crystallizationprocess, thereby increasing crystallinity. It therefore becomes possibleto set the driving frequency of the source signal line driver circuit to10 MHZ or higher.

[0157] First, a TFT having a structure in which hot carrier injection isreduced without decreasing the operating speed as much as possible isused as an n-channel TFT of a CMOS circuit forming the driver circuitportion. Note that the driver circuit referred to here includes circuitssuch as a shift register, a buffer, a level shifter, a latch inline-sequential drive, and a transmission gate in dot-sequential drive.

[0158] In Embodiment 3, the active layer of the n-channel TFT containsthe source region, the drain region, the LDD region overlapping with thegate electrode with the gate insulating film sandwiched therebetween(Lov region), the LDD region not overlapping with the gate electrodewith the gate insulating film sandwiched therebetween (Loff region), andthe channel forming region.

[0159] Further, there is not much need to worry about degradation due tothe hot carrier injection with the p-channel TFT of the CMOS circuit,and therefore LDD regions may not be formed in particular. It is ofcourse possible to form LDD regions similar to those of the n-channelTFT, as a measure against hot carriers.

[0160] In addition, when using a CMOS circuit in which electric currentflows in both directions in the channel forming region, namely a CMOScircuit in which the roles of the source region and the drain regioninterchange, it is preferable that LDD regions be formed on both sidesof the channel forming region of the n-channel TFT forming the CMOScircuit, sandwiching the channel forming region. A circuit such as atransmission gate used in dot-sequential drive can be given as anexample of such. Further, when a CMOS circuit in which it is necessaryto suppress the value of the off current as much as possible is used,the n-channel TFT forming the CMOS circuit preferably has an Lov region.A circuit such as the transmission gate used in dot-sequential drive canbe given as an example of such.

[0161] Note that, in practice, it is preferable to perform packaging(sealing), without exposure to the atmosphere, using a protecting film(such as a laminated film or an ultraviolet cured resin film) havinggood airtight properties and little outgassing, or a transparent sealingmaterial, after completing through the state of FIG. 10B. At this time,the reliability of the EL element is increased by making an inertatmosphere on the inside of the sealing material and by arranging adrying agent (barium oxide, for example) inside the sealing material.

[0162] Further, after the airtight properties have been increased by thepackaging process, a connector (flexible printed circuit: FPC) isattached in order to connect terminals led from the elements or circuitsformed on the substrate with external signal terminals. Then, a finishedproduct is completed. This state at which the product is ready forshipment is referred to as a display device throughout thisspecification.

[0163] Furthermore, in accordance with the process shown in Embodiment3, the number of photo masks required for manufacture of a displaydevice can be suppressed. As a result, the process can be shortened, andthe reduction of the manufacturing cost and the improvement of the yieldcan be attained.

[0164] (Embodiment 4)

[0165]FIG. 11A is a top surface diagram of an EL display device usingthe driving method of the present invention. In FIG. 11A, referencenumeral 4010 denotes a substrate, while reference numeral 4011 denotes apixel portion, 4012 denotes a source signal line driver circuit, and4013 denotes a gate signal line driver circuit. The respective drivercircuits are connected to an external equipment via wirings 4014 and4016 leading to an FPC 4017.

[0166] A cover material 6000, an airtight sealing material (alsoreferred to as a housing material) 7000, and a sealing material (asecond sealing material) 7001 are provided at this time so as tosurround at least the pixel portion, and preferably the driver circuitand the pixel portion.

[0167] Further, FIG. 11B is a cross sectional structure of the ELdisplay device of Embodiment 4, and a driver circuit TFT (note that aCMOS circuit in which an n-channel TFT and a p-channel TFT are combinedis shown in the figures here) 4022 and a pixel portion TFT 4023 (notethat only an EL driving TFT is shown in the figures here) are formed ona base film 4010 on the substrate 4021. Known structures (top gatestructures or bottom gate structures) may be used for these TFTs.

[0168] After completing the driver circuit TFT 4022 and the pixelportion TFT 4023 by using a known method of manufacturing, a pixelelectrode 4027 made from a transparent conducting film for electricallyconnecting to a drain of the pixel portion TFT 4023 is formed on aninterlayer insulating film (leveling film) 4026 made from a resinmaterial. A compound of indium oxide and tin oxide (referred to as ITO)and a compound of indium oxide and zinc oxide can be used as thetransparent conducting film. An insulating film 4028 is formed once thepixel electrode 4027 is formed, and an open portion is formed on thepixel electrode 4027.

[0169] An EL layer 4029 is formed next. A lamination structure of aknown EL material (hole injecting layer, hole transporting layer, lightemitting layer, electron transporting layer, and electron injectinglayer), or a single layer structure, may be used for the EL layer 4029.Further, there are low molecular weight materials and high molecularweight materials (polymer materials) for the EL material. An evaporationmethod is used when a low molecular weight material is used, but it ispossible to use a simple method such as printing or spin coating ofink-jet printing when a high molecular weight material is used.

[0170] The EL layer is formed by evaporation using a shadow mask inEmbodiment 4. Color display becomes possible by forming light emittinglayers (a red color light emitting layer, a green color light emittinglayer, and a blue color light emitting layer) capable of emitting lightat different wavelength for each pixel using the shadow mask. Inaddition, a method of combining a color changing layer (CCM) and a colorfilter, and a method of combining a white color light emitting layer anda color filter are available, and both may be used. Of course, a singlecolor light emitting electronic device can also be made.

[0171] After forming the EL layer 4029, a cathode 4030 is formed on theEL layer. It is preferable to remove as much moisture and oxygen aspossible from the interface between the cathode 4030 and the EL layer4029. A method in which the EL layer 4029 and the cathode 4030 areformed in succession within a vacuum, or in which the EL layer 4029 isformed in an inert environment and the cathode 4030 is then formedwithout exposure to the atmosphere is therefore necessary. The abovefilm formation can be performed by using a multi-chamber method (clustertool method) film formation apparatus.

[0172] Note that a lamination structure of a LiF (lithium fluoride) filmand an Al (aluminum) film is used as the cathode 4030 in Embodiment 4.Specifically, a 1 nm thick LiF (lithium fluoride) film is formed byevaporation on the EL layer 4029, and a 300 nm thick aluminum film isformed on the LiF film. An MgAg electrode, which is a known cathodematerial, may of course also be used. The cathode 4030 is then connectedto the wiring 4016 in a region denoted by reference numeral 4031. Thewiring 4016 is an electric power source supply line for applying apredetermined voltage to the cathode 4030, and is connected to the FPC4017 through a conducting paste material 4032.

[0173] The cathode 4030 and the wiring 4016 are electrically connectedin the region shown by reference numeral 4031, and therefore it isnecessary to form contact holes in the interlayer insulating film 4026and in the insulating film 4028. These contact holes may be formedduring etching of the interlayer insulating film 4026 (when the pixelelectrode contact hole is formed) and during etching of the insulatingfilm 4028 (when forming the open portion before forming the EL layer).Further, etching may also be performed together through to theinterlayer insulating film 4026 when etching the insulating film 4028. Acontact hole having a good shape can be formed in this case providedthat the interlayer insulating film 4026 and the insulating film 4028are formed by the same resin material.

[0174] A passivation film 6003, a filler material 6004 and the covermaterial 6000 are formed covering the surface of the EL element thusformed.

[0175] In addition, the sealing material 7000 is formed on the inside ofthe cover material 6000 and the substrate 4010 so as to surround the ELelement portion. The airtight sealing material (the second sealingmaterial) 7001 is formed on the outside of the sealing material 7000.

[0176] The filler material 6004 functions as an adhesive for bonding thecover material 6000. PVC (polyvinyl chloride), epoxy resin, siliconeresin, PVB (polyvinyl butyral) and EVA (ethylene vinyl acetate) can beused as the filler material 6004. A moisture absorption effect can bemaintained if a drying agent is formed on the inside of the fillermaterial 6004, and therefore it is preferable to do so.

[0177] Furthermore, spacers may be included within the filler material6004. The spacers may be made from a powdered substance composed of amaterial such as BaO, giving the spacers themselves moisture absorbency.

[0178] The passivation film 6003 can relieve the spacer pressure forcases of forming the spacers. Further, a film such as a resin film,separate from the passivation film 6003, may also be formed forrelieving the spacer pressure.

[0179] Further, a glass plate, an aluminum plate, a stainless steelplate, an FRP (fiberglass-reinformed plastic) plate, a PVF (polyvinylfluoride) film, a mylar film, a polyester film, and an acrylic film canbe used as the cover material 6000. Note that when using PVB or EVA asthe filler material 6004, it is preferable to use a sheet having astructure in which several 10 of μm of aluminum foil is sandwiched by aPVF film or a mylar film.

[0180] Note that, depending upon the direction of light emitted from theEL elements (light emission direction), it may be necessary for thecover material 6000 to have light transmitting characteristics.

[0181] Further, the wiring 4016 is electrically connected to the FPC4017 through a gap between the sealing material 7000 and the airtightsealing material 7001, and the substrate 4010. Note that, although thewiring 4016 is explained here, the other wiring 4014 are alsoelectrically connected to the FPC 4017 by passing under the sealingmaterial 7000 and the airtight sealing material 7001.

[0182] Note that the cover material 6000 is bonded after forming thefiller material 6004 in FIG. 11, and that the sealing material 7000 isattached so as to the side surface (exposed surface) of the fillermaterial 6004, but the filler material 6004 may also be formed afterattaching the cover material 6000 and the sealing material 7000. Afiller material injection port passing through the gap formed by thesubstrate 4010, the cover material 6000 and the sealing material 7000 isformed in this case. The gap is then placed in a vacuum state (equal toor less than 10⁻² torr), and after immersing the injection port in atank containing the filler material, the pressure on the outside of thegap is made higher than the pressure within the gap, and the fillermaterial fills the space.

[0183] (Embodiment 5)

[0184] Next, an example of manufacturing the EL display device whichhave a different form from that shown in FIGS. 11A and 11B is explainedusing FIGS. 12A and 12B. The explanation of the same number as FIGS. 11Aand 11B are omitted because they are indicated same portion.

[0185]FIG. 12A is a top view of an EL display device using the presentinvention. FIG. 12B shows a cross sectional view which is cut along theline A-A′ in FIG. 12A.

[0186] A passivation film 6003 is formed covering the surface of the ELelement thus made according to FIG. 11.

[0187] The filler material 6004 is provided and further functions as anadhesive for bonding the cover member 6000. PVC (polyvinyl chloride),epoxy resin, silicone resin, PVB (polyvinyl butyral), and EVA (ethylenevinyl acetate) can be used as the filler material 6004. If a dryingagent is formed on the inside of the filler material 6004, then it cancontinue to maintain a moisture absorbing effect, which is preferable.

[0188] Further, spacers may be contained within the filler material6004. The spacers may be a powdered substance such as BaO, giving thespacers themselves the ability to absorb moisture.

[0189] When using spacers, the passivation film 6003 can relieve thespacer pressure. Further, a film such as a resin film can be formedseparately from the passivation film to relieve the spacer pressure.

[0190] Furthermore, a glass plate, an aluminum plate, a stainless steelplate, an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinylfluoride) film, a Mylar film, a polyester film, and an acrylic film canbe used as the cover material 6000. Note that if PVB or EVA is used asthe filler material 6004, it is preferable to use a sheet with astructure in which several tens of μm of aluminum foil is sandwiched bya PVF film or a Mylar film.

[0191] However, depending upon the light emission direction from the ELelement (the light radiation direction), it is necessary for the covermaterial 6000 to have light transmitting characteristics.

[0192] Next, the cover material 6000 is bonded by using the fillermaterial 6004. Thereafter, a frame member 6001 is attached so as tocover side surfaces (exposed surfaces) formed by the filler material6004. The frame member 6001 is bonded by a sealing material 6002(functioning as an adhesive). Preferably, a photo-setting resin is usedas sealing material 6002. However, a thermosetting resin may be used ifthe heat resistance of the EL layer is high enough to allow use of sucha resin. It is desirable that the sealing material 6002 has suchproperties as to inhibit permeation of moisture and oxygen aseffectively as possible. A desiccant may be mixed in the sealingmaterial 6002.

[0193] Further, the wiring 4016 is electrically connected to the FPC4017 through a gap between the sealing material 6002 and the substrate4010. Note that although an explanation of the wiring 4016 has been madehere, the wiring 4014 is also electrically connected to the FPC 4017 bysimilarly passing underneath the sealing material 6002.

[0194] In FIG. 12, the cover material 6000 is bonded after forming thefiller material 6004, and the frame material 6001 is attached so as tocover the lateral surfaces (exposed surfaces) of the filler material6004, but the filler material 6004 may also be formed after attachingthe cover material 6000 and the frame material 6001. In this case, aninjection opening of the filler material is formed through a gap formedby the substrate 4010, the cover material 6000, and the frame material6001. The gap is set into a vacuum state (a pressure equal to or lessthan 10⁻² Torr), and after immersing the injection opening in the tankholding the filler material, the air pressure outside of the gap is madehigher than the air pressure within the gap, and the filler materialfills the gap.

[0195] (Embodiment 6)

[0196] Here, FIG. 13 illustrates a further detailed structure in crosssection of a pixel portion of an EL display device. In FIG. 13, aswitching TFT 4502 provided on a substrate 4501 is an n-channel type TFTformed by a known method. In the present embodiment, the switching TFT4502 is of a double gate structure with gate electrodes 39 a and 39 b.By adopting the double gate structure, two TFTs are substantiallyconnected in series, and thus, there is an advantage that an off currentvalue can be decreased. It is to be noted that, though the double gatestructure is adopted in the present embodiment, a single gate structure,a triple gate structure, or a multiple gate structure having more thanthree gates may also be adopted. Further, a p-channel type TFT formed bya known method may also be used.

[0197] In the present embodiment, an EL driving TFT 4503 is an n-channeltype TFT formed by a known method. A gate electrode 37 of the EL drivingTFT 4503 is electrically connected to a drain wiring 35 of the switchingTFT 4502 via a wiring 36.

[0198] Since the EL driving TFT is an element for controlling the amountof electric current through the EL element, a lot of electric currentpasses through it, and thus, it is highly liable to deterioration due toheat or due to hot carrier. Therefore, a structure, in which an LDDregion is provided at the side of a drain of the EL driving TFT so as tooverlap the gate electrode through a gate insulating film, is quiteeffective.

[0199] Further, a single gate structure with one gate electrode 37 ofthe EL driving TFT 4503 is shown in the figures in this embodiment, buta multi-gate structure in which a plurality of TFTs are connected inseries may also be used. In addition, a structure in which a pluralityof TFTs are connected in parallel, with partition into a plurality ofchannel forming regions, and which can perform radiation of heat withhigh efficiency, may also be used.

[0200] Though the top gate TFT is used in this embodiment, the bottomgate TFT can also be used.

[0201] Further, a source wiring 40 is connected to a power supply line(not illustrated), and constant voltage is always applied to the sourcewiring 40.

[0202] A first passivation film 41 is formed on the switching TFT 4502and the EL driving TFT 4503, and a leveling film 42 comprising aninsulating resin film is formed on the first passivation film 41. It isextremely important to level the step due to the TFTs using the levelingfilm 42. An EL layer formed later is extremely thin, and there are casesin which defective light emissions occur. Therefore, to form the ELlayer with its surface which is as level as possible, it is preferableto perform leveling before forming a pixel electrode.

[0203] Furthermore, reference numeral 43 denotes a pixel electrode (acathode of an EL element in this case) made from a conductive film withhigh reflectivity, and is electrically connected to a drain region ofthe EL driving TFT 4503. It is preferable to use a low resistanceconductive film, such as an aluminum alloy film, a copper alloy film,and a silver alloy film, or a laminate of such films. Of course, alamination structure with another conductive film may also be used.

[0204] In addition, a light emitting layer 45 is formed in a groove(corresponding to a pixel) formed by banks 44 a and 44 b formed ofinsulating films (preferably resins). Note that only one pixel is shownin the figure here, but the light emitting layer may correspond to eachof the colors R (red), G (green), and B (blue). A π conjugate polymermaterial is used as an organic EL material. Polyparaphenylene vinylenes(PPVs), polyvinyl carbazoles (PVKs), and polyfluoranes can be given astypical polymer materials.

[0205] Note that there are several types of PPV organic EL materials,and materials recorded in Shenk, H., Becker, H., Gelsen, O., Kluge, E.,Kreuder, W., and Spreitzer, H., “Polymers for Light Emitting Diodes”,Euro Display Proceedings, 1999, pp. 33-7, and in Japanese PatentApplication Laid-open No. Hei 10-92567, for example, may be used.

[0206] As specific light emitting layers, cyano-polyphenylene vinylenemay be used as a red light emitting layer, polyphenylene vinylene may beused as a green light emitting layer, and polyphenylene vinylene orpolyalkylphenylene may be used as a blue light emitting layer. The filmthickness may be between 30 and 150 nm (preferably between 40 and 100nm).

[0207] However, the above example is merely one example of the organicEL materials which can be used as light emitting layers, and it is notnecessary to limit use to these materials. An EL layer may be formed byfreely combining light emitting layers, electron transport layers, andelectron injection layers.

[0208] For example, the present embodiment shows an example of using apolymer material as a light emitting layer, but a low molecular weightorganic EL material may also be used. Further, it is possible to useinorganic materials such as silicon carbide, as an electron transportlayer or an electron injection layer. Known materials can be used forthese organic EL materials and inorganic materials.

[0209] An EL layer with a laminate structure, in which a hole injectionlayer 46 made of PEDOT (polythiophene) or PAni (polyaniline) is formedon the light emitting layer 45, is used in the present embodiment. Ananode 47 is then formed on the hole injection layer 46 of a transparentconductive film. The light generated in the light emitting layer 45 isradiated toward the upper surface (the opposite direction to thesubstrate 4501 where TFT is formed) in the present embodiment, andtherefore the anode must have a conductive property and be formed of amaterial with a property of being transparent to light. A compound ofindium oxide and tin oxide, or a compound of indium oxide and zinc oxidecan be used as the transparent conductive film. However, because it isformed after forming the low heat resistance light emitting and holeinjection layers, it is preferable to use a material which can bedeposited at as low a temperature as possible.

[0210] An EL element 4505 is complete at the point where the anode 47 isformed. Note that what is called the EL element 4505 here is formed bythe pixel electrode (anode) 43, the light emitting layer 45, the holeinjection layer 46, and the anode 47. The pixel electrode 43 is nearlyequal in area to the pixel, and consequently the entire pixel functionsas an EL element. Therefore, the light emitting efficiency is extremelyhigh, and a bright image display becomes possible.

[0211] In addition, a second passivation film 48 is then formed on theanode 47 in the present embodiment. It is preferable to use a siliconnitride film or an oxidized silicon nitride film as the secondpassivation film 48. The purpose of this is the isolation of the ELelement from the outside, and this is meaningful in preventingdegradation due to oxidation of the organic EL material, and incontrolling gaseous emitted from the organic EL material. Thereliability of the EL display can thus be raised.

[0212] The EL display device of the present invention has a pixelportion made from pixels structured as in FIG. 13, and has a switchingTFT with a sufficiently low off current value, and a current control TFTwhich is strong with respect to hot carrier injection. An EL device withhigh reliability, and in which good image display is possible, cantherefore be obtained.

[0213] (Embodiment 7)

[0214] In this embodiment, there will be described a structure in whichthe structure of the EL element 4505 is reversed in the pixel portionillustrated in Embodiment 6. Explanation will be given with reference toFIG. 14. Note that, since the points of difference from the structureshown in FIG. 13 lie only in the EL element and the driver TFT, theother explanation shall be omitted from description.

[0215] Referring to FIG. 14, an EL driving TFT 4503 is formed using thep-channel TFT manufactured by known method.

[0216] In this embodiment, a transparent conductive film is employed asa pixel electrode (anode) 50. Concretely, the conductive film is made ofa compound of indium oxide and zinc oxide. Of course, a conductive filmmade of a compound of indium oxide and tin oxide may well be employed.

[0217] Besides, after banks 51 a and 51 of an insulating film have beenformed, a light emitting layer 52 made of polyvinylcarbazole is formedon the basis of the application of a solution. An electron injectionlayer 53 made of potassium acetylacetonate (expressed as acacK), and acathode 54 made of an aluminum alloy are formed thereon. In this case,the cathode 54 functions also as a passivation film. Thus, an EL element4701 is formed.

[0218] In the case of this embodiment, light generated in the lightemitting layer 52 is radiated toward a substrate 4501 formed with TFTsas indicated by an arrow.

[0219] (Embodiment 8)

[0220] The following description of Embodiment 8 is the construction ofthe source signal line driver circuit.

[0221]FIG. 6 is a circuit diagram showing the source signal line drivercircuit. A shift register 8801, a latch (A) 8802 and a latch (B) 8803are disposed as shown in FIG. 6. In Embodiment 8, the latch (A) 8802 andthe latch (B) 8803 are disposed to correspond to four source signallines S_a to S_d. Embodiment 8 is not provided with a level shifter forvarying the amplitude width of the voltage of a signal, the level shiftmay be disposed as required.

[0222] A clock signal CLK, a clock signal CLKB which has an oppositepolarity to the clock signal CLK, a start pulse signal SP and adriving-direction switching signal SL/R are inputted to the shifterregister 8801 via the respective lines shown in FIG. 6. A digital signalVD which is inputted from the outside is divided into four signals, andthe four signals are inputted to the latch (A) 8802 via the respectivelines shown in FIG. 6. A latch signal S_LAT and a signal S_LATh whichhas an opposite polarity to the latch signal S_LAT are inputted to thelatch (B) 8803 via the respective lines shown in FIG. 6.

[0223] When the signal output from the shifter register 8801 is inputtedto the latch (A) 8802, the latch (A) 8802 obtains four signals at thesame time from the four divided digital signals VD. In response to thelatch signals S_LAT and S_LATh, the latch (B) 8803 latches the digitalsignals VD and output them to the source signal lines S_a to S_d.

[0224] In the above description of Embodiment 8, the method ofsimultaneously sampling signals corresponding to four source signallines with the use of four divided video signals. However, in general,n-number of divided digital signals may be used to simultaneously samplesignals corresponding to n-number of source signal lines.

[0225] The detailed construction of the latch (A) 8802 will be describedbelow with illustrative reference to a part 8804 of the latch (A) 8802which corresponds to the source signal line S_a. The part 8804 of thelatch (A) 8802 has two clocked inverters and two inverters.

[0226]FIG. 7 is a top plan view showing a part 8804 of the latch (A)8802. Reference numerals 831 a and 831 b denote active layers of TFTswhich form one of the inverters of the part 8804 of the latch (A) 8802,and reference numeral 836 denotes a gate electrode which is common tothe TFTs which form the one of the inverters. Reference numerals 832 aand 832 b denote active layers of TFTs which form the other of theinverters of the part 8804 of the latch (A) 8802, and reference numerals837 a and 837 b denote gate electrodes which are provided over theactive layers 832 a and 832 b, respectively. Incidentally, the gateelectrodes 837 a and 837 b are electrically connected to each other.

[0227] Reference numerals 833 a and 833 b denote active layers of TFTswhich form one of the clocked inverters of the part 8804 of the latch(A) 8802. Gate electrodes 838 a and 838 b are provided over the activelayer 833 a so as to constitute a double-gate structure. The gateelectrode 838 b and a gate electrode 839 are provided over the activelayer 833 b so as to constitute a double-gate structure.

[0228] Reference numerals 834 a and 834 b denote active layers of TFTswhich form the other of the clocked inverters of the part 8804 of thelatch (A) 8802. The gate electrode 839 and a gate electrode 840 areprovided over the active layer 834 a so as to constitute a double-gatestructure. The gate electrode 840 and a gate electrode 841 are providedover the active layer 834 b so as to constitute a double-gate structure.

[0229] (Embodiment 9)

[0230] Embodiment 9 will be described below with reference to FIGS. 15Aand 15B which show a fabricated example of an EL display device usingthe driving method according to the invention. FIG. 15A is a top planview showing the state in which EL elements formed on an active matrixsubstrate are sealed. Sections 6801, 6802 and 6803, each of which isshown by dashed lines, are a source signal line driver circuit, a gatesignal line driver circuit and a pixel portion, respectively. Sections6804, 6805 and 6806 are a cover material, a first sealing material and asecond sealing material, respectively. A filler 6807 (refer to FIG. 15B)is provided in the inside portion surrounded by the first sealingmaterial 6805 between the cover material 6804 and the active matrixsubstrate.

[0231] Reference numeral 6808 denotes connecting lines to transmit inputsignals to the source signal line driver circuit 6801, the gate signalline driver circuit 6802 and the pixel portion 6803, and the connectinglines 6808 receive video signals and clock signals from an FPC (flexibleprinted circuit) 6809 which serves as a connecting terminal to externalequipment.

[0232]FIG. 15B is a cross-sectional view taken along line A-A′ of FIG.15A. In FIGS. 15A and 15B, the same reference numerals are used todenote the same portions.

[0233] As shown in FIG. 15B, the pixel portion 6803 and the sourcesignal line driver circuit 6801 are formed on a substrate 6800, and thepixel portion 6803 is made of plural pixels each including a TFT 6851for controlling current to flow through an EL element (hereinafterreferred to as the EL driving TFT 6851), a pixel electrode 6852electrically connected to a drain region of the EL driving TFT 6851 andthe like. In Embodiment 9, the EL driving TFT 6851 is a p-channel typeTFT. The source signal line driver circuit 6801 is formed of a CMOScircuit in which an N-channel type TFT 6853 and a P-channel type TFT6854 are complementarily combined with each other.

[0234] Each pixel has a color filter (R) 6855, a color filter (G) 6856or a color filter (B) (not shown) under its pixel electrode. The colorfilter (R) is a color filter which extracts red light, the color filter(G) is a color filter which extracts green light, and the color filter(B) is a color filter which extracts blue light. The color filter (R)6855 is provided in a pixel which emits red, the color filter (G) 6856is provided in a pixel which emits green, and the color filter (B) isprovided in a pixel which emits blue.

[0235] The first advantage of the case where these color filters areprovided is that the color purity of each emitted color is improved. Forexample, red light is emitted from the EL element of each pixel whichemits red (toward the pixel electrode in the present embodiment), andthe purity of red can be improved by passing the red light through thecolor filter which extracts red light. The other green light and bluelight are also subjected to similar processing.

[0236] In a conventional structure which does not use color filters,there may occur the problem that visible light which enters from theoutside of an EL display device excites the emitting layers of its ELelements and no desired colors can be obtained. However, if colorfilters are disposed as in the case of Embodiment 9, light withparticular wavelength is only allowed to enter the EL elements. That isto say, it is possible to prevent the problem that the EL elements areexcited by external light.

[0237] Incidentally, although structures provided with color filtershave heretofore been proposed, white-emitting EL elements have been usedin such structures. In this case, light with the other wavelengths iscut off to extract red light, so that a lowering of luminance isincurred. However, in Embodiment 9, since red light emitted from the ELelements is passed through color filters which extract red light, alowering of luminance is prevented from being incurred.

[0238] The pixel electrode 6852 is formed of a transparent conductivefilm, and functions as the anode of the EL element. Insulating films6857 are formed at both ends of the pixel electrode 6852, and inaddition, an emitting layer 6858 which emits red light and an emittinglayer 6859 which emits green light are formed. Incidentally, althoughnot shown, an emitting layer which emits blue light is provided in anadjacent pixel, whereby color display is provided by the pixels whichindividually correspond to red, green and blue. Of course, the pixelscomprising blue-emitting layers are provided with color filters whichextract blue light.

[0239] Not only an organic material but also an inorganic material maybe used as an EL material. In addition, a stacked structure, in which anelectron injection layer, an electron transport layer, a hole transportlayer and a hole injection layer are combined, may be adopted.

[0240] A cathode 6860 of the EL element is formed of a conductive filmwith light-shielding characteristics, on each of the emitting layers.This cathode 6860 is common to all the pixels, and is electricallyconnected to the FPC 6809 via connecting lines 6808.

[0241] Then, the first sealing material 6805 is formed with a dispenseror the like, and spacers (not shown) are scattered and the covermaterial 6804 is stuck. Then, the area which is surrounded by the activematrix substrate 6800, the cover material 6804 and the first sealingmaterial 6805 is filled with the filler 6807 by a vacuum injectionmethod.

[0242] In addition, in Embodiment 9, barium oxide is previously added tothe filler 6807 as a hygroscopic material 6861. Incidentally, inEmbodiment 9, the filler 6807 is a filler containing a hygroscopicmaterial, but the hygroscopic material may also be sealed in the fillerin the state of being dispersed in massive form. Although not shown, ahygroscopic material may also be used as the material of spacers.

[0243] Then, after the filler 6807 has been cured by irradiation ofultraviolet rays or by heating, an opening (not shown) formed in thefirst sealing material 6805 is closed. After the openings of the firstsealing material 6805 have been closed, the connecting lines 6808 andthe FPC 6809 are electrically connected to each other by the use of aconductive material 6862. In addition, a second sealing material 6806 isformed to cover the exposed portion of the first sealing material 6805and a part of the FPC 6809. The second sealing material 6806 may use thesame material as the first sealing material 6805.

[0244] By sealing the EL elements with filler 6807 with the use of theabove-described method, it is possible to completely isolate the ELelements from the outside, whereby a substance which promotes oxidationof an organic material, such as water or oxygen, can be prevented frompenetrating from the outside. Accordingly, it is possible to fabricate ahighly reliable EL display device.

[0245] (Embodiment 10)

[0246] In the description of Embodiment 10, an example will bedescribed, in which the direction of irradiation of light emitted fromEL elements and the arrangement of color filters differ in the ELdisplay device according to Embodiment 9. The following description usesFIG. 16, but the pixel portion shown in FIG. 16 is the same in basicstructure as that shown in FIG. 15B, and the modified portions of theconstruction shown in FIG. 16 are denoted by different referencenumerals.

[0247] As shown in FIG. 15B, a pixel portion 6901 is formed of pluralpixels each including a TFT 6902 for controlling current to flow throughthe EL element (hereinafter referred to as the EL driving TFT 6902), apixel electrode 6903 electrically connected to a drain region of the TFT6902 and the like.

[0248] In Embodiment 10, an n-channel type TFT is used as the EL drivingTFT 6902 in the pixel portion 6901. The pixel electrode 6903 iselectrically connected to the drain of the EL driving TFT 6902, and isformed of a conductive film with light-shielding characteristics. InEmbodiment 10, the pixel electrode 6903 serves as a cathode of the ELelement.

[0249] A transparent conductive film 6904 which is common to all thepixels is formed on the light emitting layer 6858 which emits red lightand the light emitting layer 6859 which emits green light. Thetransparent conductive film 6904 serves as an anode of the EL element.

[0250] In addition, Embodiment 10 has the feature that a color filter(R) 6905, a color filter (G) 6906 and a color filter (B) (not shown) areformed on the cover material 6804. In the case of the structure of theEL element according to Embodiment 10, the direction of irradiation oflight emitted from the EL layer is toward the cover material 6804,whereby the color filters can be disposed in the path of light in thestructure shown in FIG. 16.

[0251] In the case of forming the color filter (R) 6905, the colorfilter (G) 6906 and the color filter (B) (not shown) on the covermaterial 6804, there is the advantage that it is possible to reduce thenumber of the steps required to fabricate the active matrix substrateand it is possible to realize an improvement in yield factor andthroughput.

[0252] (Embodiment 11)

[0253] In the EL display device using the driving method according tothe present invention, the material for the EL layers of EL elements isnot limited to organic EL materials, and may also use inorganic ELmaterials. However, since the current inorganic EL materials need veryhigh driving voltages, it is necessary to use TFTs which have breakdownvoltage characteristics against such driving voltages.

[0254] In addition, if inorganic EL materials with lower driving voltageare developed in the future, such inorganic EL materials can be appliedto the present invention.

[0255] (Embodiment 12)

[0256] In the EL display device using the driving method according tothe present invention, organic materials to be used for the EL layer maybe low molecular weight organic materials or polymeric (high molecularweight) organic materials. Materials such as Alq₃(tris-8-quinolinolato-aluminum) and TPD (triphenylamine derivative) areknown as the low molecular weight organic materials. As the polymericorganic materials, π-conjugated polymeric materials are used.Representative examples are PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole) and polycarbonate.

[0257] The polymeric (high molecular weight) organic materials can beformed by a simple thin-film deposition method such as a spin-coatingmethod (also called a solution-applying method), a dipping method, adispensing method, a printing method, or an ink-jet method, and havehigher heat resistance then molecular weight organic materials.

[0258] In the EL element of an EL display device, if the EL layer of theEL element has an electron transport layer and a hole transport layer,the electron transport layer and the hole transport layer may be formedof an amorphous semiconductor of an inorganic material such as amorphousSi or amorphous Si_(1-x)C_(x).

[0259] In the amorphous semiconductor, a large number of trap levels arepresent, and a large number of interfacial levels are formed at theinterface where the amorphous semiconductor is in contact with anotherlayer. Accordingly, the EL elements can emit light at low voltage, and afar higher luminance can be realized.

[0260] Dopants (impurities) may also be added to an organic EL layer tochange the color of light to be emitted from the organic EL layer. Thedopants are DCM 1, nile red, rubrene coumarin 6, TPB, quinacridone andthe like.

[0261] (Embodiment 13)

[0262] This embodiment will be described on electronic devicesincorporated an EL display device using the driving method of theinvention.

[0263] As these electronic devices, there can be enumerated a videocamera, a digital camera, a head-mountable display (a goggle typedisplay), a game machine, a car navigation, a personal computer, and amobile information terminal (e.g., a mobile computer, a mobile telephoneor an electronic book), as shown in FIGS. 18A to 18E.

[0264]FIG. 18A shows a personal computer including a body 2001, a casing2002, a display portion 2003 and a keyboard 2004. The EL display deviceusing a driving method of the present invention can be used as thedisplay portion 2003 of the personal computer.

[0265]FIG. 18B shows a video camera including a body 2101, a displayportion 2102, a voice input unit 2103, manipulation switches 2104, abattery 2105 and an image receiving unit 2106. The EL display deviceusing a driving method of the present invention can be used as thedisplay portion 2102 of the video camera.

[0266]FIG. 18C shows one portion (i.e., a right-hand side) of ahead-mounted display including a body 2301, a signal cable 2302, a headfixing band 2303, a display unit 2304, an optical system 2305 and adisplay portion 2306. The EL display device using a driving method ofthe present invention can be used the display portion 2306 of thehead-mounted display.

[0267]FIG. 18D shows an image reproducing device (e.g., a DVDreproducing device) provided with a recording medium. The imagereproducing device includes a body 2401, a recording medium (CD, LD orDVD and the like) 2402, manipulation switches 2403 and display units (a)2404 and (b) 2405. The display portion 2404 (a) displays a imageinformation and the display portion (b) 2405 displays characterinformation. The EL display device using a driving method of the presentinvention can be used the display portion (a) 2404 and (b) 2405. Here,this device is enabled to CD reproduction device and the game device asthe recording medium.

[0268]FIG. 18E shows a mobile computer including a body 2501, a cameraportion 2502, a image receiving unit 2503, an operation switch 2504 anda display portion 2505. The EL display device using a driving method ofthe present invention can be used as the display portion 2505 of themobile computer.

[0269] As has been described hereinbefore, the invention can have anextremely wide range of applications and can be applied to electronicdevices of any fields. On the other hand, the electronic device of thisembodiment can be realized by using a construction of any of thecombinations of Embodiments 1 to 12.

[0270] In the related art gray scale display method for active matrixtype EL display devices, there has been the problem that the amount ofcurrent which flows in the EL elements of an EL display device becomesnon-uniform due to the unevenness of the characteristics of the TFTs ofthe pixel portion of the EL display device or due to variations in theenvironmental temperature during the use of the EL display device, sothat unevenness occurs in the luminance display of the EL displaydevice.

[0271] However, owing to the above-described construction, the inventionmakes it possible to keep a current which flows in each of the ELelements of the pixel portion, constant with respect to variations intemperature, thereby suppressing the unevenness of display. Accordingly,it is possible to provide a driving method for an EL display devicecapable of high-quality display.

What is claimed is:
 1. A method of driving a display device whichcomprises a pixel comprising an EL element and a transistor, comprisingthe step of: dividing one frame period into plural sub-frame periods,and applying one of a first gate voltage and a second gate voltage to agate electrode of the transistor during each of the plural sub-frameperiods, wherein a drain current of the transistor flows between bothelectrodes of the EL element to place the EL element into an emittingstate when the first gate voltage is applied to the gate electrode ofthe transistor, wherein the transistor is placed into a non-conductivestate and the EL element is placed into a non-emitting state when thesecond gate voltage is applied to the gate electrode of the transistor,and wherein an absolute value of the first gate voltage is not greaterthan an absolute value of a voltage across a drain and a source of thetransistor.
 2. A method of driving a display device which comprises apixel comprising an EL element, a transistor and a resistor, comprisingthe step of: dividing one frame period into plural sub-frame periods,and applying one of a first gate voltage and a second gate voltage to agate electrode of the transistor during each of the plural sub-frameperiods, wherein a drain current of the transistor flows across theresistor and both electrodes of the EL element and the EL element isplaced into an emitting state when the first gate voltage is applied tothe transistor, wherein the transistor is placed into a non-conductivestate and the EL element is placed into a non-emitting state when thesecond gate voltage is applied to the gate electrode of the transistor,and wherein an absolute value of the first gate voltage is not greaterthan an absolute value of a voltage across a drain and a source of thetransistor.
 3. A method of driving a display device according to claim1, wherein as the ratio of a gate width to a gate length of thetransistor is smaller than 1, the absolute value of the first gatevoltage applied to the gate electrode of the transistor is largerwithout exceeding the absolute value of the voltage across the drain andthe source of the transistor.
 4. A method of driving a display deviceaccording to claim 1, wherein the EL element enables color display byusing an EL layer which emits light of one color in combination with acolor conversion layer.
 5. A method of driving a display deviceaccording to claim 1, wherein the EL element enables color display byusing an EL layer which emits white light, in combination with a colorfilter.
 6. A method of driving a display device according to claim 1,wherein an EL layer of the EL element comprises one of a low molecularweight organic material and a polymeric organic material.
 7. A method ofdriving a display device according to claim 6, wherein the low molecularweight organic material is one of Alq₃ (tris-8-quinolinolato-aluminum)and TPD (triphenylamine derivative).
 8. A method of driving a displaydevice according to claim 6, wherein the polymeric organic material isone of PPV (polyphenylene vinylene), PVK (poly(vinylcarbazole), andpolycarbonate.
 9. A method of driving a display device according toclaim 1, wherein the EL layer of the EL element comprises an inorganicmaterial.
 10. A video camera, an image reproducing apparatus, ahead-mounted display, a mobile telephone or a mobile informationterminal which uses the method of driving a display device according toclaim
 1. 11. A method of driving a display device according to claim 2,wherein as the ratio of a gate width to a gate length of the transistoris smaller than 1, the absolute value of the first gate voltage appliedto the gate electrode of the transistor is larger without exceeding theabsolute value of the voltage across the drain and the source of thetransistor.
 12. A method of driving a display device according to claim2, wherein the EL element enables color display by using an EL layerwhich emits light of one color in combination with a color conversionlayer.
 13. A method of driving a display device according to claim 2,wherein the EL element enables color display by using an EL layer whichemits white light, in combination with a color filter.
 14. A method ofdriving a display device according to claim 2, wherein an EL layer ofthe EL element comprises one of a low molecular weight organic materialand a polymeric organic material.
 15. A method of driving a displaydevice according to claim 14, wherein the low molecular weight organicmaterial is one of Alq₃ (tris-8-quinolinolato-aluminum) and TPD(triphenylamine derivative).
 16. A method of driving a display deviceaccording to claim 14, wherein the polymeric organic material is one ofPPV (polyphenylene vinylene), PVK (poly(vinylcarbazole), andpolycarbonate.
 17. A method of driving a display device according toclaim 2, wherein the EL layer of the EL element comprises an inorganicmaterial.
 18. A video camera, an image reproducing apparatus, ahead-mounted display, a mobile telephone or a mobile informationterminal which uses the method of driving a display device according toclaim
 2. 19. A method of driving a display device which comprises apixel comprising an EL element and a transistor, comprising the step of:dividing one frame period into plural sub-frame periods, and applyingone of a first gate voltage and a second gate voltage to a gateelectrode of the transistor during each of the plural sub-frame periods,wherein a drain current of the transistor flows between both electrodesof the EL element to place the EL element into an emitting state whenthe first gate voltage is applied to the gate electrode of thetransistor, wherein the transistor is placed into a non-conductive stateand the EL element is placed into a non-emitting state when the secondgate voltage is applied to the gate electrode of the transistor, whereinan absolute value of the first gate voltage is not greater than anabsolute value of a voltage across a drain and a source of thetransistor, and wherein as the ratio of a gate width to a gate length ofthe transistor is smaller than 1, the absolute value of the first gatevoltage applied to the gate electrode of the transistor is largerwithout exceeding the absolute value of the voltage across the drain andthe source of the transistor.
 20. A method of driving a display deviceaccording to claim 19, wherein the EL element enables color display byusing an EL layer which emits light of one color in combination with acolor conversion layer.
 21. A method of driving a display deviceaccording to claim 19, wherein the EL element enables color display byusing an EL layer which emits white light, in combination with a colorfilter.
 22. A method of driving a display device according to claim 19,wherein an EL layer of the EL element comprises one of a low molecularweight organic material and a polymeric organic material.
 23. A methodof driving a display device according to claim 22, wherein the lowmolecular weight organic material is one of Alq₃(tris-8-quinolinolato-aluminum) and TPD (triphenylamine derivative). 24.A method of driving a display device according to claim 22, wherein thepolymeric organic material is one of PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole), and polycarbonate.
 25. A method of driving adisplay device according to claim 19, wherein the EL layer of the ELelement comprises an inorganic material.
 26. A video camera, an imagereproducing apparatus, a head-mounted display, a mobile telephone or amobile information terminal which uses the method of driving a displaydevice according to claim
 19. 27. A method of driving a display devicewhich comprises a pixel comprising an EL element and a transistor,comprising the step of: dividing one frame period into plural sub-frameperiods, and applying one of a first gate voltage and a second gatevoltage to a gate electrode of the transistor during each of the pluralsub-frame periods, wherein the EL element is placed into an emittingstate when the first gate voltage is applied to the gate electrode ofthe transistor, wherein the EL element is placed into a non-emittingstate when the second gate voltage is applied to the gate electrode ofthe transistor, and wherein an absolute value of the first gate voltageis not greater than an absolute value of a voltage across a drain and asource of the transistor.
 28. A method of driving a display device whichcomprises a pixel comprising an EL element, a transistor and a resistor,comprising the step of: dividing one frame period into plural sub-frameperiods, and applying one of a first gate voltage and a second gatevoltage to a gate electrode of the transistor during each of the pluralsub-frame periods, wherein the EL element is placed into an emittingstate when the first gate voltage is applied to the transistor, whereinthe EL element is placed into a non-emitting state when the second gatevoltage is applied to the gate electrode of the transistor, and whereinan absolute value of the first gate voltage is not greater than anabsolute value of a voltage across a drain and a source of thetransistor.
 29. A method of driving a display device according to claim27, wherein as the ratio of a gate width to a gate length of thetransistor is smaller than 1, the absolute value of the first gatevoltage applied to the gate electrode of the transistor is largerwithout exceeding the absolute value of the voltage across the drain andthe source of the transistor.
 30. A method of driving a display deviceaccording to claim 27, wherein the EL element enables color display byusing an EL layer which emits light of one color in combination with acolor conversion layer.
 31. A method of driving a display deviceaccording to claim 27, wherein the EL element enables color display byusing an EL layer which emits white light, in combination with a colorfilter.
 32. A method of driving a display device according to claim 27,wherein an EL layer of the EL element comprises one of a low molecularweight organic material and a polymeric organic material.
 33. A methodof driving a display device according to claim 32, wherein the lowmolecular weight organic material is one of Alq₃(tris-8-quinolinolato-aluminum) and TPD (triphenylamine derivative). 34.A method of driving a display device according to claim 32, wherein thepolymeric organic material is one of PPV (polyphenylene vinylene), PVK(poly(vinylcarbazole), and polycarbonate.
 35. A method of driving adisplay device according to claim 27, wherein the EL layer of the ELelement comprises an inorganic material.
 36. A video camera, an imagereproducing apparatus, a head-mounted display, a mobile telephone or amobile information terminal which uses the method of driving a displaydevice according to claim
 27. 37. A method of driving a display deviceaccording to claim 28, wherein as the ratio of a gate width to a gatelength of the transistor is smaller than 1, the absolute value of thefirst gate voltage applied to the gate electrode of the transistor islarger without exceeding the absolute value of the voltage across thedrain and the source of the transistor.
 38. A method of driving adisplay device according to claim 28, wherein the EL element enablescolor display by using an EL layer which emits light of one color incombination with a color conversion layer.
 39. A method of driving adisplay device according to claim 28, wherein the EL element enablescolor display by using an EL layer which emits white light, incombination with a color filter.
 40. A method of driving a displaydevice according to claim 28, wherein an EL layer of the EL elementcomprises one of a low molecular weight organic material and a polymericorganic material.
 41. A method of driving a display device according toclaim 40, wherein the low molecular weight organic material is one ofAlq₃ (tris-8-quinolinolato-aluminum) and TPD (triphenylaminederivative).
 42. A method of driving a display device according to claim40, wherein the polymeric organic material is one of PPV (polyphenylenevinylene), PVK (poly(vinylcarbazole), and polycarbonate.
 43. A method ofdriving a display device according to claim 28, wherein the EL layer ofthe EL element comprises an inorganic material.
 44. A video camera, animage reproducing apparatus, a head-mounted display, a mobile telephoneor a mobile information terminal which uses the method of driving adisplay device according to claim
 28. 45. A method of driving a displaydevice which comprises a pixel comprising an EL element and atransistor, comprising the step of: dividing one frame period intoplural sub-frame periods, and applying one of a first gate voltage and asecond gate voltage to a gate electrode of the transistor during each ofthe plural sub-frame periods, wherein the EL element is placed into anemitting state when the first gate voltage is applied to the gateelectrode of the transistor, wherein the EL element is placed into anon-emitting state when the second gate voltage is applied to the gateelectrode of the transistor, wherein an absolute value of the first gatevoltage is not greater than an absolute value of a voltage across adrain and a source of the transistor, and wherein as the ratio of a gatewidth to a gate length of the transistor is smaller than 1, the absolutevalue of the first gate voltage applied to the gate electrode of thetransistor is larger without exceeding the absolute value of the voltageacross the drain and the source of the transistor.
 46. A method ofdriving a display device according to claim 45, wherein the EL elementenables color display by using an EL layer which emits light of onecolor in combination with a color conversion layer.
 47. A method ofdriving a display device according to claim 45, wherein the EL elementenables color display by using an EL layer which emits white light, incombination with a color filter.
 48. A method of driving a displaydevice according to claim 45, wherein an EL layer of the EL elementcomprises one of a low molecular weight organic material and a polymericorganic material.
 49. A method of driving a display device according toclaim 48, wherein the low molecular weight organic material is one ofAlq₃ (tris-8-quinolinolato-aluminum) and TPD (triphenylaminederivative).
 50. A method of driving a display device according to claim48, wherein the polymeric organic material is one of PPV (polyphenylenevinylene), PVK (poly(vinylcarbazole), and polycarbonate.
 51. A method ofdriving a display device according to claim 45, wherein the EL layer ofthe EL element comprises an inorganic material.
 52. A video camera, animage reproducing apparatus, a head-mounted display, a mobile telephoneor a mobile information terminal which uses the method of driving adisplay device according to claim 45.